Quenched steel sheet having excellent strength and ductility and method for manufacturing same

ABSTRACT

Disclosed are a quenched steel sheet and a method for manufacturing the same. The quenched steel sheet according to an aspect of the present invention contains, in terms of wt %, C: 0.05˜0.25%, Si: 0.5% or less (excluding  0 ), Mn: 0.1˜2.0%, P: 0.05% or less, S: 0.03% or less, the remainder Fe, and other unavoidable impurities, wherein a refined structure of the steel sheet comprises 90 volume % or more of martensite with a first hardness and martensite with a second hardness.

RELATED APPLICATIONS

This application is the Divisional of U.S. patent application Ser. No.15/101,384 filed on Jun. 2, 2016 which is the U.S. National Phase under35 U.S.C. § 371 of International Application No. PCT/KR2013/012132 filedon Dec. 4, 2013, which in turn claims the benefit of Korean PatentApplication No. 10-2013-0161430 filed on Dec. 23, 2013, the disclosureof which applications are incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a quenched steel plate havingexcellent strength and ductility and a method of manufacturing the same.

BACKGROUND ART

In terms of steel, strength and ductility are inversely related, and thefollowing technologies according to the related art are used as methodsof obtaining steel having excellent strength and ductility.

As representative examples, there are technologies of controlling aphase fraction of a ferrite, bainite, or martensite structure such asdual phase (DP) steel disclosed in Korean Patent Publication No.0782785, transformation induced plasticity (TRIP) steel disclosed inKorean Patent Publication No. 0270396, as well as controlling a residualaustenite fraction by utilizing an alloying element such as manganese(Mn), nickel (Ni), or the like disclosed in Korean Patent PublicationNo. 1054773.

However, in a case of DP steel or TRIP steel, increases in strength arelimited to 1200 MPa. In addition, in the case of a technology ofcontrolling a residual austenite fraction, increases in strength arelimited to 1200 MPa, and there may be a problem of increasedmanufacturing costs due to the addition of a relatively expensivealloying element.

Thus, the development of a steel in which relatively expensive alloyingelements may be used in significantly reduced amounts and excellentstrength and ductility may be provided is required.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a quenched steel sheethaving excellent strength and ductility without adding a relativelyexpensive alloying element by properly controlling an alloy compositionand heat treatment conditions, and a method of manufacturing the same.

Technical Solution

According to an aspect of the present disclosure, a quenched steel sheetmay be a steel plate including, by wt %, carbon (C): 0.05% to 0.25%,silicon (Si): 0.5% or less (with the exception of 0), manganese (Mn):0.1% to 2.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less,iron (Fe) as a residual component thereof, and other unavoidableimpurities. The quenched steel sheet may include 90 volume % or more ofmartensite having a first hardness and martensite having a secondhardness as a microstructure of the steel plate. The first hardness mayhave a greater hardness value than a hardness value of the secondhardness, and a ratio of a difference between the first hardness and thesecond hardness and the first hardness may satisfy relational expression1.

5≤(first hardness-second hardness)/(first hardness)*100≤30  [RelationalExpression 1]

According to another aspect of the present disclosure, a quenched steelsheet may be a quenched steel sheet provided by cold rolling and heattreating a steel plate including, by wt %, carbon (C): 0.05% to 0.25%,silicon (Si): 0.5% or less (with the exception of 0), manganese (Mn):0.1% to 2.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less,iron (Fe) as a residual component thereof, and other unavoidableimpurities, and including ferrite and pearlite as a microstructure. Themicrostructure of the quenched steel sheet includes 90 volume % or moreof martensite having a first hardness and martensite having a secondhardness. The martensite having the first hardness is provided throughtransformation occurring from pearlite before heat treatment and in aregion adjacent to the pearlite before heat treatment, and themartensite having the second hardness is provided through transformationoccurring from ferrite before heat treatment and in a region adjacent tothe ferrite before heat treatment.

According to another aspect of the present disclosure, a method ofmanufacturing a quenched steel sheet according to an exemplaryembodiment in the present disclosure may include: cold rolling a steelplate including, by wt %, carbon (C): 0.05% to 0.25%, silicon (Si): 0.5%or less (with the exception of 0), manganese (Mn): 0.1% to 2.0%,phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less, iron (Fe) as aresidual component thereof, and other unavoidable impurities, andincluding ferrite and pearlite as a microstructure at a reduction ratioof 30% or more; heating the cold-rolled steel plate to a heatingtemperature (T*) of Ar3° C. to Ar3+500° C.; and cooling the heated steelplate. A heating rate (v_(r), ° C./sec) satisfies relational expression2 when heating the steel plate, and a cooling rate (v_(c), ° C./sec)satisfies relational expression 3 when cooling the steel plate.

v _(r)≥(T*/110)²  [Relational Expression 2]

v _(c)≥(T*/80)²  [Relational Expression 3]

Advantageous Effects

According to an exemplary embodiment in the present disclosure, aquenched steel sheet having excellent strength and ductility, of which atensile strength is 1200 MPa or more and elongation is 7% or morewithout adding a relatively expensive alloying element, may be provided.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a microstructure of a steel plate before heattreatment, observed with an electron microscope, according to anexemplary embodiment in the present disclosure.

FIG. 2 illustrates a microstructure, observed with an opticalmicroscope, of a steel plate after heat treatment of inventive example 4meeting conditions of an exemplary embodiment in the present disclosure.

FIG. 3 illustrates a microstructure, observed with an opticalmicroscope, of a steel plate after heat treatment of comparative example5 under conditions other than those of an exemplary embodiment in thepresent disclosure.

BEST MODE FOR INVENTION

The inventors have conducted research to solve problems of the abovedescribed related art. As a result, a carbon content may be properlyprovided and cold rolling and a heat treatment process may be properlycontrolled in the present disclosure, thereby forming two kinds ofmartensite having different levels of hardness as a microstructure of asteel plate. Thus, a steel plate capable of having improved strength andductility without adding a relatively expensive alloying element may beprovided.

Hereafter, a quenched steel sheet having excellent strength andductility according to an exemplary embodiment in the present disclosurewill be described in detail. In the present disclosure, ‘heat treatment’means heating and cooling operations carried out after cold rolling.

First, an alloy composition of a quenched steel sheet according to anexemplary embodiment in the present disclosure is described in detail.

Carbon (C): 0.05 wt % to 0.25 wt %

Carbon is an essential element for improving the strength of a steelplate, and carbon may be required to be added in a proper amount tosecure martensite which is required to be implemented in the presentdisclosure. In a case in which the content of C is less than 0.05 wt %,it may be difficult not only to obtain sufficient strength of a steelplate, but also to secure a martensite structure of 90 volume % or moreas a microstructure of a steel plate after heat treatment. On the otherhand, in a case in which the content of C exceeds 0.25 wt %, ductilityof the steel plate may be decreased. In the present disclosure, thecontent of C may be properly controlled within a range of 0.05 wt % to0.25 wt %.

Silicon (Si): 0.5 wt % (with the Exception of 0)

Si may serve as a deoxidizer, and may serve to improve strength of asteel plate. In a case in which the content of Si exceeds 0.5 wt %,scale may be formed on a surface of the steel plate in a case in whichthe steel plate is hot-rolled, thereby degrading surface quality of thesteel plate. In the present disclosure, the content of Si may beproperly controlled to be 0.5 wt % or less (with the exception of 0).

Manganese (Mn): 0.1 wt % to 2.0 wt %

Mn may improve strength and hardenability of steel, and Mn may becombined with S, inevitably contained therein during a steelmanufacturing process to then form MnS, thereby serving to suppress theoccurrence of crack caused by S. In order to obtain the effect in thepresent disclosure, the content of Mn may be 0.1 wt % or more. On theother hand, in a case in which the content of Mn exceeds 2.0 wt %,toughness of steel may be decreased. In the present disclosure, thus,the content of Mn may be controlled to be within a range of 0.1 wt % to2.0 wt %.

Phosphorus (P): 0.05 wt % or Less

P is an impurity inevitably contained in steel, and P is an element thatis a main cause of decreasing ductility of steel as P is organized in agrain boundary. Thus, a content of P may be properly controlled to be asrelatively low. Theoretically, the content of P may be advantageouslylimited to be 0%, but P is inevitably provided during a manufacturingprocess. Thus, it may be important to manage an upper limit thereof. Inthe present disclosure, an upper limit of the content of P may bemanaged to be 0.05 wt %.

Sulfur (S): 0.03 wt % or Less

S is an impurity inevitably contained in steel, and S is an element tobe a main cause of increasing an amount of a precipitate due to MnSformed as S reacts to Mn, and of embrittling steel. Thus, a content of Smay be controlled to be relatively low. Theoretically, the content of Smay be advantageously limited to be 0%, but S is inevitably providedduring a manufacturing process. Thus, it may be important to manage anupper limit. In the present disclosure, an upper limit of the content ofS may be managed to be 0.03 wt %.

The quenched steel sheet may also include iron (Fe) as a remainderthereof, and unavoidable impurities. On the other hand, the addition ofan active component other than the above components is not excluded.

Hereinafter, a microstructure of a quenched steel sheet according to anexemplary embodiment in the present disclosure will be described indetail.

A quenched steel sheet according to an exemplary embodiment in thepresent disclosure may satisfy a component system, and may include 90volume % or more of martensite having a first hardness and martensitehaving a second hardness as a microstructure of a steel plate. In a casein which two kinds of martensite are less than 90 volume %, it may bedifficult to sufficiently secure required strength. Meanwhile, accordingto an exemplary embodiment in the present disclosure, the remainder ofmicrostructures, other than the martensite structure, may includeferrite, pearlite, cementite, and bainite.

According to an exemplary embodiment in the present disclosure, thequenched steel sheet is a steel plate manufactured by cold rolling andheat treating a steel plate including ferrite and pearlite as amicrostructure. The martensite having the first hardness may be obtainedby being transformed from pearlite before heat treatment and in a regionadjacent thereto, and the martensite having the second hardness may beobtained by being transformed from ferrite before heat treatment and ina region adjacent thereto. As described later in the present disclosure,in a case in which heat treatment conditions of a cold-rolled steelplate are properly controlled, the diffusion of carbon may besignificantly reduced, thereby forming two kinds of martensite asdescribed above.

In a case in which such a structure is secured as the microstructure ofthe steel plate, first transformation may occur in martensite havingrelatively low hardness in an initial process. As subsequenttransformation proceeds, work hardening may occur, thereby improvingductility of the steel plate. In order to obtain the above effectaccording to an exemplary embodiment in the present disclosure, a ratioof a difference between the first hardness and the second hardness andthe first hardness may be properly controlled to satisfy relationalexpression 1. In a case in which the ratio thereof is less than 5%, aneffect of improving ductility of the steel plate may be insufficient,while in a case in which the ratio thereof exceeds 30%, transformationmay be concentrated on an interface of structures of two kinds ofmartensite, whereby a crack may occur. Thus, ductility of the steelplate may be decreased.

(first hardness−second hardness)/(first hardness)*100≥30  [RelationalExpression 1]

Meanwhile, according to an exemplary embodiment in the presentdisclosure, an average packet size of the two kinds of martensite may be20 μm or less. In a case in which the packet size exceeds 20 μm, since ablock size and a plate size inside a martensite structure are increasedsimultaneously, strength and ductility of the steel plate may bedecreased. Thus, the packet size of the two kinds of martensite may beproperly controlled to be 20 μm or less.

Hereafter, according to another exemplary embodiment in the presentdisclosure, a method of manufacturing a quenched steel sheet havingexcellent strength and ductility will be described in detail.

The steel plate satisfying the afore-described composition and includingferrite and pearlite as a microstructure may be cold-rolled. Asdescribed above, ferrite and pearlite are sufficiently secured as amicrostructure of a steel plate before heat treatment. In a case inwhich heat treatment conditions are properly controlled, two kinds ofmartensite having different levels of hardness after heat treatment maybe formed.

In a case in which the steel plate is cold rolled, a reduction ratiothereof may be 30% or more. As described above, in a case in which thesteel plate is cold-rolled at a reduction ratio of 30% or more, as aferrite structure is elongated in a rolling direction, a relativelylarge amount of residual transformation may be included inside thereof.In addition, as a pearlite structure is also elongated in a rollingdirection, a fine carbide may be formed therein. The cold-rolled ferriteand pearlite structures may allow an austenite grain to be refined in acase in which subsequent heat treatments are undertaken, and mayfacilitate employment of a carbide. Thus, strength and ductility of thesteel plate may be improved. Meanwhile, FIG. 1 is a view illustrating amicrostructure, observed with an electron microscope, of a steel platebefore heat treatment according to an exemplary embodiment in thepresent disclosure. It can be confirmed in FIG. 1 that ferrite andpearlite structures are elongated in a rolling direction, and a finecarbide is formed inside the pearlite structure.

Next, the cold-rolled steel plate is heated to a heating temperature(T*) of Ar3° C. to Ar3+500° C. For example, in a case in which theheating temperature (T*) is less than Ar3° C., austenite may not besufficiently formed. Thus, a martensite structure of 90 volume % or moremay not be obtained after cooling the steel plate. On the other hand, ina case in which the heating temperature (T*) exceeds Ar3° C.+500° C., anaustenite grain may be coarsened, and diffusion of carbon may beaccelerated. Thus, two kinds of martensite having different levels ofhardness may not be obtained after cooling the steel plate. Thus, theheating temperature may be Ar3° C. to Ar3+500° C., and in detail, beAr3° C. to Ar3+300° C.

In a case in which heating the steel plate, a heating rate (v_(r), °C./sec) may satisfy the following relational expression 2. If the v_(r)does not satisfy relational expression 2, an austenite grain iscoarsened during heating of the steel plate, and carbon is excessivelydiffused. Thus, two kinds of martensite having different hardness maynot be obtained after cooling the steel plate. Meanwhile, as a heatingrate is increased, an austenite grain is prevented from being coarsenedand carbon is prevented from being diffused. Thus, an upper limitthereof is not particularly limited.

v _(r)≥(T*/110)²  [Relational Expression 2]

Meanwhile, according to an exemplary embodiment in the presentdisclosure, the cold-rolled and heated steel plate may have an austenitesingle phase structure having an average diameter of 20 μm or less as amicrostructure thereof. In a case in which an average diameter of theaustenite single phase structure exceeds 20 μm, there may be a risk ofcoarsening a packet size of a martensite structure formed after coolingthe steel plate, and there may be a risk of decreasing strength andductility of the steel plate by increasing a martensite transformationtemperature.

Next, the heated steel plate is cooled. In this case, a cooling rate(v_(c), ° C./sec) may satisfy the following relational expression 3. Ifthe v_(c) does not satisfy relational expression 3, an austenite grainis coarsened during cooling of the steel plate, and carbon isexcessively diffused. Thus, two kinds of martensite having differenthardness may not be obtained after cooling the steel plate. In addition,a structure of the steel plate may be transformed into a ferrite,pearlite, or bainite structure during cooling of the steel plate. Thus,it may be difficult to secure a targeted martensite volume fraction.Meanwhile, as the cooling rate is increased, an austenite grain may beprevented from being coarsened and carbon may be prevented from beingdiffused. Thus, an upper limit thereof is not particularly limited.

V _(c)≥(T*/80)²  [Relational Expression 3]

Meanwhile, according to an exemplary embodiment in the presentdisclosure, in a case in which cooling the heated steel plate, ahigh-temperature retention time (t_(m), sec) may satisfy the followingrelational expression 4. The high-temperature retention time means thetime required for initiating cooling of a steel plate having reached aheating temperature. In a case in which the high-temperature retentiontime satisfies relational expression 4, carbon may be prevented frombeing excessively diffused, and in addition, since an average diameterof an austenite grain before cooling is controlled to be 20 μm or less,martensite having an average packet size of 20 μm or less after coolingmay be secured. Meanwhile, as the high-temperature retention time isfurther decreased, an austenite grain may be prevented from beingcoarsened and carbon from being diffused. Thus, a lower limit thereof isnot particularly limited.

t _(m)≤(8−0.006*T*)²  [Relational Expression 4]

Hereinafter, the exemplary embodiments in the present disclosure will bedescribed in more detail. The present disclosure may, however, beexemplified in many different forms and should not be construed as beinglimited to the specific embodiments set forth herein. While exemplaryembodiments are shown and described, it will be apparent to thoseskilled in the art that modifications and variations could be madewithout departing from the scope of the present invention as defined bythe appended claims below.

Embodiment

After steel plates having compositions illustrated in Table 1 isprepared, the steel plates were cold-rolled, heated, and cooled in acondition of Table 2. Then, a microstructure of the steel plate wasobserved, mechanical properties were measured, and the results therefromare shown in Table 3. In this case, a tensile test was performed at arate of 5 mm/min with respect to an ASTM subsized specimen, and aVickers hardness test of each microstructure was performed at acondition in which the microstructure was maintained at a load of 5 gfor 10 seconds.

TABLE 1 Steels C Mn Si P S Comparative 0.04 0.17 0.005 0.01 0.005 Steel1 Inventive 0.10 1.49 0.003 0.02 0.003 Steel 1 Inventive 0.21 0.89 0.0050.015 0.012 Steel 2

TABLE 2 Reduction T* t_(m) t_(m)* Steels Ratio (%) (° C.) v_(r) (°C./sec) v_(r)* (° C./sec) v_(c) (° C./sec) v_(c)* (° C./sec) (sec) (sec)Note Comparative 70 1000 300 83 1000 156 1 4 Comparative Steel 1 Example1 Comparative 70 900 300 67 1000 126 1 6.8 Comparative Steel 1 Example 2Inventive 60 700 300 40 1000 76 1 14 Comparative Steel 1 Example 3Inventive 60 900 300 67 1000 126 1 6.8 Inventive Steel 1 Example 1Inventive 60 1000 300 82 1000 156 1 4 Inventive Steel 1 Example 2Inventive 70 900 300 67 1000 126 1 6.8 Inventive Steel 2 Example 3Inventive 70 1000 300 83 1000 156 1 4 Inventive Steel 2 Example 4Inventive 70 1100 300 100 1000 189 1 2 Inventive Steel 2 Example 5Inventive 70 1200 300 119 1000 225 0.1 0.6 Inventive Steel 2 Example 6Inventive 70 1000 200 83 1000 156 1 4 Inventive Steel 2 Example 7Inventive 70 1000 100 83 1000 156 1 4 Inventive Steel 2 Example 8Inventive 70 1000 300 83 200 156 1 4 Inventive Steel 2 Example 9Inventive 70 1000 300 83 1000 156 2 4 Inventive Steel 2 Example 10Inventive 70 1000 50 83 1000 156 1 4 Comparative Steel 2 Example 4Inventive 70 700 300 40 1000 76 1 14 Comparative Steel 2 Example 5Inventive 70 1000 300 83 1000 156 5 4 Comparative Steel 2 Example 6Inventive 70 1000 300 83 1000 156 20 4 Comparative Steel 2 Example 7Inventive 70 1000 300 83 80 156 1 4 Comparative Steel 2 Example 8Inventive 70 1200 300 119 1000 225 1 0.6 Comparative Steel 2 Example 9Inventive 70 1300 300 140 1000 264 1 0.04 Comparative Steel 2 Example 10v_(r)* is a heating rate ((T*/110)²) calculated by relational expression2, v_(c)* is a cooling rate ((T*/80)²) calculated by relationalexpression 3, and t_(m)* is a high-temperature retention time ((8 −0.006 * T*)²) calculated by relational expression 4.

TABLE 3 First Second Tensile Micro- hardness hardness Relational PacketStrength Elongation Steels structure (HV) (HV) expression 1 Size (μm)(MPa) (%) Note Comparative F + P — — — — 655 11.1 Comparative Steel 1Example 1 Comparative F + P — — — — 661 17.8 Comparative Steel 1 Example2 Inventive F + P — — — — 1014 11.9 Comparative Steel 1 Example 3Inventive M1 + M2 454 372 28.1 8.9 1347 8.2 Inventive Steel 1 Example 1Inventive M1 + M2 437 368 25.8 12.2 1311 9.7 Inventive Steel 1 Example 2Inventive M1 + M2 662 513 22.5 6.8 1795 7.4 Inventive Steel 2 Example 3Inventive M1 + M2 650 520 20 8.5 1775 8.1 Inventive Steel 2 Example 4Inventive M1 + M2 627 510 23.7 13.7 1771 7.7 Inventive Steel 2 Example 5Inventive M1 + M2 619 526 25.1 16.7 1702 8.1 Inventive Steel 2 Example 6Inventive M1 + M2 634 513 19.1 11.8 1763 7.3 Inventive Steel 2 Example 7Inventive M1 + M2 607 549 9.6 10.7 1742 7.1 Inventive Steel 2 Example 8Inventive M1 + M2 614 545 11.2 9.1 1711 7.2 Inventive Steel 2 Example 9Inventive M1 + M2 631 560 11.2 9.6 1759 7.2 Inventive Steel 2 Example 10Inventive M1 + M2 567 540 4.7 15.5 1687 6.4 Comparative Steel 2 Example4 Inventive F + P — — — — 1387 3.2 Comparative Steel 2 Example 5Inventive M1 + M2 591 563 4.8 19.7 1712 5.9 Comparative Steel 2 Example6 Inventive M1 + M2 578 553 4.3 27.7 1699 2.9 Comparative Steel 2Example 7 Inventive F + P — — — — 649 20.1 Comparative Steel 2 Example 8Inventive M1 + M2 570 543 22.1 4.7 1689 6.7 Comparative Steel 2 Example9 Inventive M1 + M2 559 536 28.9 4.1 1684 6.4 Comparative Steel 2Example 10 Here, F is ferrite, P is pearlite, M1 is martensite having afirst hardness, and M2 is martensite having a second hardness

Inventive examples 1 to 10, satisfying a composition and a manufacturingmethod according to an exemplary embodiment in the present disclosure,include two kinds of martensite, a hardness difference of which isbetween 5% to 30%, thereby having tensile strength of 1200 MPa or moreand elongation of 7% or more.

Meanwhile, comparative examples 1 and 2 include ferrite and pearlite asa microstructure after heat treatment as a carbon content in steel isrelatively low, and strength thereof is inferior.

In addition, in comparative example 3, since a heating temperature (T*)is relatively low, ferrite and pearlite are included as a microstructureafter heat treatment, and strength thereof is inferior. In comparativeexample 5, a heating temperature (T*) is relatively low, but a carboncontent is relatively high. Thus, strength of steel is in a rangecontrolled according to an exemplary embodiment in the presentdisclosure. However, a rolling structure by cold rolling is notsufficiently loosened, whereby ductility thereof is inferior.

In addition, in comparative examples 4, 6, 7, 9, and 10, one of v_(r)and t_(m) is outside of a range controlled according to an exemplaryembodiment in the present disclosure. Thus, an austenite grain iscoarsened, and carbon is diffused, whereby a martensite structure inwhich a difference of hardness is less than 5% is formed. In addition,steel strength is excellent, but ductility thereof is inferior.

In addition, in comparative example 8, v_(c) is outside of a rangecontrolled according to an exemplary embodiment in the presentdisclosure. Ferrite and pearlite structures are formed during coolingthe steel plate, and ductility thereof is excellent but strength isinferior.

Meanwhile, FIG. 2 is a view illustrating a microstructure of a steelplate after heat treatment, observed with an optical microscope,according to inventive example 4 of the present disclosure. FIG. 3 is aview illustrating a microstructure of a steel plate after heattreatment, observed with an optical microscope, according to comparativeexample 5. Referring to FIG. 2, in a case of inventive example 4, a sizeof a martensite packet is finely formed to be 20 μm or less. Thus, aplate inside the packet is also finely formed. Meanwhile, referring toFIG. 3 illustrating comparative example 5, a size of a martensite packetexceeds 20 μm, and thus, martensite is formed to be coarse. In addition,a plate inside the packet is also formed to be coarse.

1. A method for manufacturing a quenched steel sheet comprising: coldrolling a steel plate comprising, by wt %, carbon (C): 0.05% to 0.25%,silicon (Si): 0.5% or less (with the exception of 0), manganese (Mn):0.1% to 2.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.03% or less,iron (Fe) as a residual component thereof, and other unavoidableimpurities, and comprising ferrite and pearlite as a microstructure at areduction ratio of 30% or more; heating the cold-rolled steel plate to aheating temperature (T*) of Ar3° C. to Ar3+500° C.; and cooling theheated steel plate, wherein in the heating of the cold-rolled steelplate, a heating rate (v_(r), ° C./sec) satisfies relational expression2, and in the cooling of the heated steel plate, a cooling rate (v_(c),° C./sec) satisfies relational expression 3,v _(r)≥(T*/110)²  [Relational Expression 2]v _(c)≥(T*/80)².  [Relational Expression 3]
 2. The method formanufacturing a quenched steel sheet of claim 1, wherein in the coolingof the heated steel plate, the high-temperature retention time (t_(m))satisfies relational expression 4,t _(m)(8−0.006*T*)².  [Relational Expression 4]
 3. The method formanufacturing a quenched steel sheet of claim 1, wherein themicrostructure of the steel plate is an austenite single phase structurehaving an average diameter of 20 μm or less.